1. Field of the Invention
The invention relates to a power supply input circuit comprising a disconnecting circuit coupled to receive an AC-input voltage and a disconnecting signal for disconnecting the AC-input voltage from output terminals of the disconnecting circuit if the disconnecting signal is active, a series arrangement of a first and a second smoothing capacitor coupled between output terminals of the power supply input circuit, a voltage doubler circuit coupled to said output terminals for supplying a rectified output voltage across the series arrangement of the first and the second smoothing capacitor in a first situation where no voltage doubling is required, and for supplying one polarity of the AC-input voltage across said first smoothing capacitor and the other polarity of the AC-input voltage across said second smoothing capacitor, to provide a doubled rectified output voltage across the series arrangement of the first and the second smoothing capacitor in a second situation where a voltage doubling is required, and an over-voltage detection circuit having an output for supplying the disconnecting signal.
The invention also relates to an over-voltage protection method of protecting smoothing capacitors in a voltage doubler, the voltage doubler receiving an AC-input voltage and comprising a series arrangement of two smoothing capacitors coupled between output terminals of the voltage doubler, while in a first situation where no voltage doubling is required, the voltage doubler supplies a rectified output voltage across the series arrangement of the two smoothing capacitors, and in a second situation where a voltage doubling is required, the voltage doubler supplies one polarity of the AC-input voltage across one of the smoothing capacitors and supplies the other polarity of the AC-input voltage across the other smoothing capacitor to provide a doubled rectified voltage across the series arrangement of the two smoothing capacitors, the method comprising the steps of: detecting an over-voltage, generating an active disconnecting signal if an over-voltage is detected, and disconnecting the AC-input voltage from the voltage doubler in response to the active disconnecting signal.
A power supply input circuit comprising a voltage doubler and an over-voltage circuit may be used, for example in apparatuses which have to operate on a large range of AC-input voltages. Low AC-input voltages being about a magnitude of two lower than high AC-input voltages may occur in this large range of AC-input voltages. Doubling the low AC-input voltages with the voltage doubler causes a smaller input voltage range of a stabilizing power supply succeeding the power supply input circuit.
2. Description of the Related Art
Such a power supply input circuit is known from JP-A-1-185,133. The known power supply input circuit comprises rectifier diodes arranged in a full-wave rectifier bridge, this bridge having two output terminals which are connected to a series arrangement of two smoothing capacitors. The voltage across the series arrangement of the two smoothing capacitors is the output voltage of the power supply input circuit. The full-wave rectifier bridge further comprises two input terminals, one of which is connected to one terminal of an AC-input voltage source via a fuse. At high AC-input voltages (for example, 220 V) the other input terminal of the full-wave rectifier bridge is connected to the other terminal of the AC-input voltage source via a switch being in a first position, to supply a full-wave rectifier AC-input voltage to the output terminals of the power supply input circuit. At low AC-input voltages (for example, 110 V) the switch is in a second position to connect the other terminal of the AC-input voltage source to a connection node of the two smoothing capacitors to supply a doubled AC-input voltage to the output terminals of the power supply input circuit (the other terminal of the full-wave rectifier bridge is not connected, and only two rectifier diodes are operational). An avalanche diode is connected in parallel to each smoothing capacitor. If the switch is erroneously in the second position at high AC-input voltages, an over-voltage on the smoothing capacitors is prevented because the avalanche diodes will start conducting, thereby causing a large short-circuit current that blows the fuse. A drawback of the known power supply input circuit is that the rectifier diodes have to cope with the large short circuit current. An over-voltage is herein understood to be a voltage higher than the rated voltage of the smoothing capacitors. The rated voltage is the maximum operating voltage the smoothing capacitors are designed for.
Such a power supply input circuit is also known from a Sanken Electric Company publication SSE-17284E which describes an application circuit of a voltage doubler Integrated Circuit STR81000A. This known circuit again comprises a fuse, rectifier diodes arranged in a full wave rectifier bridge, a semiconductor switch, and two smoothing capacitors, arranged and operating in the same way as described above. The integrated circuit comprises the switch and electronics to determine the right position of the switch depending on the level of the AC-input voltage. Over-voltage on the smoothing capacitors as a consequence of a wrong position of the switch at a high AC-input voltage is now prevented by causing a thyristor to conduct if a too high voltage across the series arrangement of the smoothing capacitors is detected. As the thyristor precedes the rectifier bridge, a large short circuit current for blowing a fuse will not flow through the rectifier diodes. Thus, this known power supply input circuit solves the problem of the first-mentioned known power supply input circuit, but has the drawback that a short circuit of only one of the smoothing capacitors is not detected. In that case a voltage (for example, 390 V) which is far higher than the rated voltage (for example, 250 V) of the smoothing capacitors will occur across the smoothing capacitor which is not short-circuited. Such a high voltage can cause a breakdown of this smoothing capacitor in a dangerous way.